Light absorption and radiation use efficiency of autumn and spring sown sugar beets

The cultivation of autumn sown sugar beets is expected to result in large yield increases due to more light absorption. The objective of this study was thus to analyse growth of autumn sown sugar beets with regard to (a) leaf development and light absorption and, (b) radiation use efficiency, the conversion of the absorbed light into dry matter. In 2005/06 and 2006/07, field trials were conducted at 4 sites with 3 sowing dates in autumn and 3 sowing dates in spring. The results show that the formation of leaf dry matter of autumn sown beets followed thermal time. A leaf area index of 3.5, and thereby canopy closure, was reached 3–4 weeks earlier than in spring sown beets resulting in extra absorption of light, in particular in April and May. Autumn sown beets produced 1.2 g dry matter per MJ solar radiation absorbed and thus, did not differ from spring sown beets in the radiation use efficiency. From light absorption and radiation use efficiency the possible yield of autumn sown sugar beets was estimated, provided these beets would not bolt after winter. Autumn sown beets could capture 2338 MJ solar radiation between sowing and harvest during October in the following year. Taking into account the improved light absorption, a theoretical yield increase of 26% compared to spring sown beets was calculated. It is discussed that yield formation of autumn sown beets would most likely be limited by the sink capacity and changes in the composition of the storage root. The data of these experiments will form a good basis for modelling yield formation of autumn sown sugar beets.